This invention relates to a method for treating emulsified latexes.
Emulsified latexes are generally in the state of dispersed solid particles of 1 .mu.m or less and since the particles are very small, industrially, it is difficult to separate and collect the solid particles as they are. Therefore, inorganic salts or acids are added to the emulsified latex or conversely the emulsified latex is added to aqueous solutions of inorganic salts or acids to once coagulate the emulsified latex and thereafter the coagulated latex is solidified by elevation of temperature and is collected as a particulate.
However, the thus obtained particulate has the following various problems to be solved.
(1) Problem of fine powders
The particulate obtained by the conventional coagulation methods is in grape cluster-like aggregations of emulsified latex particles of 1 .mu.m or less, which are aggregated by mere van der Waals forces and so have a weak binding power between the constituting particles which easily disintegrate to produce large amounts of fine powders during passing the steps. Therefore, various problems occur such as clogging of filter fabric during dehydration, reduction of washing efficiency, contamination of the working environment due to scattering of the fine powders during drying or transportation, dust explosion, etc. A large amount of equipment is required to solve these problems and furthermore dust collectors such as cyclones, bag filters, etc. must be of excessively large size. In order that the particles do not scatter and can be collected by cyclones of proper size, it is desired that the particulate does not contain fine powders of 53 .mu.m or less. Thus, development of a method for preparing particulate containing no such fine powders has been demanded.
(2) The problem of coarse particles
Production of coarse particles at coagulation causes various problems. For example, removal of impurities is difficult and quality is reduced; much air is required to fluidize the particulate during drying or transportation, water removal resistance in particles during drying reduces drying speed to necessitate a long drying time; etc. Considering these problems and problem of blending with other resins, it is generally undesirable for the particulate to contain particles of greater than 1.5 mm in diameter and desirably all particles should be of 1.0 mm or less in diameter.
(3) Increase in bulk specific gravity
When bulk specific gravity of the particulate is small, large storage vessels are required which increase storage costs and furthermore only a small amount of the particulate can be transported by one vessel to cause increase in transportation costs. Moreover, small bulk specific gravity causes scatter of particles or reduction of fluidity.
Since the particulate obtained by the conventional coagulation method is a grape cluster-like aggregation of emulsified latex particles of 1 .mu.m or less, the particulate has many voids and there is a limit in making the specific gravity higher even if coagulation conditions are variously changed. For example, in the case of rubbery resins, the limit of bulk specific gravity is 0.35 g/cm.sup.3 and it is desired to produce an integral particulate having no voids with a bulk specific gravity of 0.4 g/cm.sup.3 or more.
(4) Reduction of wet powder water content
When emulsified latex is coagulated, then dehydrated and dried to obtain a particulate, the wet powder water content at the dehydration has a direct effect on the energy required for drying. Various trials to reduce the wet powder water content, e.g., selection of dehydrators have been made, but there is a limit depending on coagulation methods. Especially in the case of grape cluster-like particulate of emulsified latex particles of 1 .mu.m or less as obtained by the conventional coagulation methods, the particulate has many voids and contains a large amount of water. Thus, it is desirable for the particulate to comprise coalescent emulsified latex particles having very few voids. For example, in the case of graft copolymer resins of polybutadiene with styrene and acrylonitrile or graft copolymer resins of polybutadiene with styrene and methylmethacrylate, particulate produced by the conventional coagulation method has a wet powder water content of more than 30% (dry base) and it is desired to further reduce this water content to save drying energy.
(5) Others
Recently, fluidity of particulate which decides the possibility of using automatic meters has become important. Especially, from the labor-saving viewpoint, there is an increasing tendency to use automatic meters in various devices while use of the automatic meters is limited in the case of the particulates produced by the conventional coagulation method. Thus, it is desired to make further labor-saving by improving fluidity of particulates. Besides there are still many problems to be solved such as particle size distribution, shape of particles, etc.
Therefore, it is desired that particulates obtained using a coagulation method contain neither fine powders of 53 .mu.m or less coarse particles of 1.5 mm or more preferably of 1.0 mm or more and having a bulk specific gravity of at least 0.4 g/cm.sup.3 and that they have a wet powder water content at dehydration of 20% or less for saving drying energy.
Many attempts have been made to solve these problems which originate from coagulation methods, but none of them have completely overcome these problems. Recently, there has been proposed to simultaneously carry out coagulation and granulation of polymer latex in a vapor phase and it has been reported that said problems were nearly completely solved. However, this method still has problems to be solved, namely, since coagulation and granulation are simultaneously effected in a vapor phase, large and complicated equipment is required, control of operations is intricate, fine powders are apt to be formed because of the injection of latex into the vapor phase, particulates of large particle diameter cannot be produced because the size of droplets at injection is the maximum of particle size of particulate obtained therefrom.
The inventors have conducted intensive research to solve these problems and attained this invention.